Genesis of Granitic Pegmatites and Enclosed Graphic Texture in the Tonglüshan Fe-Cu (Au) Skarn Deposit: Constraints from K-Feldspar ⁴⁰Ar/³⁹Ar Dating, Trace-Element Geochemistry, and Fluid Inclusion Systematics

Tonglüshan Fe-Cu (Au) deposit is one of the largest skarn deposits in middle-lower Yangtze River metallogenic belt, which is associated with the Early Cretaceous Yangxin quartz diorite stock. Granitic pegmatites are well developed in the southeast mining area, emplaced in the Yangxin quartz diorite pluton and cut by garnet-diopside skarn. The cross-cutting relationships thus indicate that the granitic pegmatites are temporarily intermediate between the quartz diorite and skarn. Granitic pegmatites consist mainly of K-feldspar, plagioclase, and quartz, with conspicuous graphic textures marked by intergrowths of K-feldspar and quartz. K-feldspar with graphic textures from one granitic pegmatite dike has been successfully dated by the ⁴⁰Ar/³⁹Ar laser microprobe incremental heating technique, yielding a well-defined plateau age of 136.0±1.0 Ma (2σ), which is interpreted to be the emplacement age of the granitic pegmatite dike. The age constraints indicate that the pegmatites formed coevally with the quartz diorite stock in the mine and related skarn Cu-Au mineralization. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) provides a wealth of information on the major and trace elements of K-feldspar from granitic pegmatites and quartz diorite. The average bulk compositions of K-feldspar from the pegmatite and quartz diorite are represented by Or₈₁Ab₁₈ and Or₇₈Ab₂₁, respectively. K-feldspar minerals with graphic textures have higher Si and lower Al than those without graphic texture. Textural and geochemical data indicate that graphic textures in the pegmatites resulted from alternating growth of K-feldspar and quartz due to dynamic alteration of the relative concentration of SiO₂ and Al₂O₃ in areas proximal to the outer zone of growing K-feldspar crystals. Both K-feldspars from quartz diorite and granitic pegmatite are enriched in large-ion lithophile elements (LILE), but the pegmatite contains more abundant Rb and Pb and lesser amounts of Ba, Sr, Li, and Cs. In the Rb-Ba, La-Ba, K/Ba-Ba, and Rb/Sr-Ba diagrams, samples of the pegmatite and quartz diorite display a trend predicted by the Rayleigh factional crystallization, indicating that the granitic pegmatite was generated by strong fractional crystallization of quartz dioritic magma. However, Pb, Li, and Ga deviate obviously from the trend of fractional crystallization, indicating these elements may have been complexed by a fluid phase. Abundance of melt and fluid inclusions in quartz minerals from the granitic pegmatites demonstrates that the pegmatite formed from melt and fluid coexisting system. This study provides a better understanding of the formation of pegmatites, exsolution of ore fluids from evolving magmas, and hydrothermal mineralization.